The use of intravascular medical devices has become an effective method for treating many types of vascular disease. In general, a suitable intravascular device is inserted into the vascular system of the patient and navigated through the vasculature to a desired target site. Using this method, virtually any target site in the patient's vascular system may be accessed, including the coronary, cerebral, and peripheral vasculature.
Medical devices such as stents, stent grafts, and vena cava filters, collectively referred to hereinafter as stents, are often utilized in combination with a delivery device for placement at a desired location within the body. A medical prosthesis, such as a stent for example, may be loaded onto a stent delivery device and then introduced into the lumen of a body vessel in a configuration having a reduced diameter. Once delivered to a target location within the body, the stent may then be expanded to an enlarged configuration within the vessel to support and reinforce the vessel wall while maintaining the vessel in an open, unobstructed condition.
Stents are generally tubular devices for insertion into body lumens. However, it should be noted that stents may be provided in a wide variety of sizes and shapes. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent may be biased so as to expand upon release from the delivery catheter and/or include a shape-memory component which allows the stent to expand upon exposure to a predetermined condition. Some stents may be characterized as hybrid stents which have some characteristics of both self-expandable and balloon expandable stents.
Stents may be constructed from a variety of materials such as stainless steel, Elgiloy, nickel, titanium, nitinol, shape memory polymers, etc. Stents may also be formed in a variety of manners as well. For example a stent may be formed by etching or cutting the stent pattern from a tube or sheet of stent material; a sheet of stent material may be cut or etched according to a desired stent pattern whereupon the sheet may be rolled or otherwise formed into the desired substantially tubular, bifurcated or other shape of the stent; one or more wires or ribbons of stent material may be woven, braided or otherwise formed into a desired shape and pattern. The density of the braid in braided stents is measured in picks per inch. Stents may include components that are welded, bonded or otherwise engaged to one another.
Flow diverting stents may treat a brain aneurysm by disrupting the flow of blood into the aneurysm using a mesh of biocompatible material placed over the aneurysm neck. Subsequently, the blood in the aneurysm stagnates and, in time, forms a thrombosis to close the aneurysm. Flow diverting stents may treat a brain aneurysm by providing resistance to blood inflow to the aneurysm. The mesh of a flow diverting stent must have sufficient pore density to disrupt the inflow to the aneurysm, but enough open area to allow side branches and perforating arteries to remain patent. To increase the therapeutic effectiveness of a flow diverting stent, the middle segment of the stent, which impedes blood flow into the aneurysm, has a low porosity.
Adjunctive neurovascular stents may treat wide necked aneurysms by providing a scaffold for retaining coils in such wide necked aneurysms. Some examples of these are the Neuroform™, Enterprise™, and the Leo™ stents. Adjunctive stents typically can be made with higher porosity (lower pore density and larger pore sizes) than flow diverting stents.
Porosity of stent material is a measure of the tendency of that material to allow passage of a fluid. A stent material's porosity index (PI) is defined as one minus the ratio of stent metal surface area to artery surface area covered by the stent. Higher porosity means that the stent material has less metal surface area compared to artery surface area and lower porosity means that the stent has more metal surface area compared to artery surface area.
Typically, a stent is implanted in a blood vessel or other body lumen at the site of a stenosis or aneurysm by so-called “minimally invasive techniques” in which the stent is compressed radially inwards and is delivered by a catheter to the site where it is required through the patient's skin or by a “cut down” technique in which the blood vessel concerned is exposed by minor surgical means. When the stent is positioned at the correct location, the stent is caused or allowed to expand to a predetermined diameter in the vessel. Many delivery devices include sheaths or catheters, and delivery members having bumpers thereon to push and pull stents through the sheaths and catheters. A catheter may be bent while navigating through torturous vasculature.
Some stents are deployed by loading them proximally from an introducer sheath into a pre-positioned microcatheter. The stent is then pushed through the microcatheter for approximately 150 cm until it is deployed from the distal end of the catheter at the treatment site. This “empty catheter” technique is different from the more traditional self-expanding stent delivery technique, which includes pre-loading the stent adjacent the distal end of the catheter and then simultaneously tracking the stent and catheter to the treatment site. The evolution of the empty catheter technique was driven by the extremely tortuous anatomy commonly found in the intracranial circulation.
First generation flow diverting stents were braided constructions of nitinol or other alloys. Typically, these have been constructed from small wire filaments around 0.0006 inches to 0.002 inches in diameter, and have between 24 and 96 wires. Braided first generation flow diverting stents work very well in many ways. They conform to tortuous anatomy well, they provide relatively uniform porosity, they are very flexible in their expanded state, and they can be reduced to relatively small profiles inside the delivery catheter.
Perceived problems with current first generation flow diversion stents include variable foreshortening of braided stents upon delivery, which makes deployed length unpredictable. Another perceived problem is increased rigidity when stents are compressed for delivery, which reduces the accuracy of deployment and the trackability of longer stents. Yet another perceived problem is the radial profile of braided stents, which reduces the size of vessel lumens. Still another perceived problem is the “fish-mouth” effect when braided stents do not expand at the distal or proximal ends, which creates difficulty in re-crossing the stent and increases thromboembolic complications at the stent ends. Another perceived problem is sub-optimal anchoring due to low opening force and the nature of braided stents, which prevents the distal end from opening independently of proximal portions. Yet another perceived problem is increasing radial profile with decreasing porosity (i.e., more braided wires), which increases stiffness and makes delivery more difficult. This perceived problem is also found in larger stents, which have more braided wires. Still another perceived problem is the unconstrained ends of braided stents, which may pose a risk to the body lumen into which the stent is deployed. Also, the pressure that delivery exerts on the ends of braided stents may also disrupt the uniformity of the braided ends upon deployment.